Los Angeles ? A federal judge on Monday temporarily barred the Navy from using a high-intensity sonar that could harm marine mammals during war games that began last week in the Pacific Ocean.

The temporary restraining order, sought by environmentalists, came three days after the Defense Department granted the Navy a six-month exemption from certain federal laws protecting marine species to allow use of the “mid-frequency active sonar.”

Environmentalists argued that the exemption was aimed at circumventing a lawsuit they filed last week to stop the Navy’s use of the sonar in the Rim of the Pacific 2006 exercise off Hawaii. The use of sonar in the war games was set to start Thursday.

In her order, U.S. District Judge Florence-Marie Cooper wrote that the environmentalists “have shown a possibility that RIMPAC 2006 will kill, injure, and disturb many marine species, including marine mammals, in waters surrounding the Hawaiian Islands.”

The Navy’s failure to take a “hard look” at the environmental impact of war games was an “arbitrary and capricious” violation of the National Environmental Policy Act, Cooper wrote.

Government lawyers were reviewing the ruling and the Navy will probably respond soon, said Jon Yoshishige, a spokesman for the U.S. Pacific Fleet in Hawaii.

The sonar exercise is intended to train sailors to detect and hunt submarines. Some wildlife authorities and advocates believe the sound waves harm whales and other mammals, possibly by damaging their hearing.

On June 27, the National Oceanic and Atmospheric Administration reached an unprecedented agreement with the Navy permitting the use of the sonar. NOAA concluded that the Navy’s use of the sonar was not likely to jeopardize the existence of threatened and endangered species – including the Hawaiian monk seal – in the exercise areas.

Cooper’s order is to remain in effect until July 18, when a hearing will take place on whether to bar use of the sonar until the lawsuit is resolved.